Efficiency to a degree is valid, yes, though in my house I've noticed an extremely negligible difference when testing antminer PSUs (though they are SUPPOSED to have significant efficiency variance).
Draw? 4x120=480W. 2x240=480W. How are you running double the rigs on the same wattage?
You're able to run more on less amp draw with 240. Not more wattage. Maybe I said that wrong. If you have 100amp service, that's your limit. If you have 200amp service, that's your limit ...
I see what you're saying, but the draw is coming from the single breaker in the main box, so it has no effect on your service limit. From that point, a 220 40a will provide enough power for 2x20a @220 or 4x20a @110. It's the same amount of energy.